The best thing we did was to not make a big deal out of it. My daughter knew that there was a "baby in my tummy" and when he came home, there were no surprises. All she said was "a baby!" and toddled off to whatever she was playing with when he came home.
We also had her help us get her brother's room ready. She was only two, but she could stack diapers on the changing table for me and fold blankets and put toys in his room. She loved it. Finally, I did get her her own "baby" complete with bottles and a diaper. When I was feeding her brother, she would hop up on the couch and feed her baby right next to me.
Bedtime can get challenging, but we made a big deal of how special it was that Dad got to put her to bed that night. They would have their own routine (hard for me to give up control!!!), but she really came to look forward to those nights. It was good for them to have their own little relationship.
Prepare her for the crying. That was about the only thing that got on my daughter's nerves. Just keep reminding her that babies cry alot and find a quiet place for her to call he own when the noise kicks in.
I hope this helps and that you have a wonderful pregnancy. Really, take it easy. I had a friend that was positively freaked out about bringing home her newborn and how it would affect her daughter. It kind of takes away from the joy of bringing baby home and I didn't want that for my family. You are going to love having another little one in the house! Good luck!